AC polarography

In AC polarography [3], as shown in Fig. 5.14(a), AC voltage (AE sin cot) of, for example, 10 mV in peak-to-peak amplitude and 100 Hz in frequency is superimposed on the DC voltage (E c) and the sum is applied to the DME. Then, from the current that flows through the circuit, the AC component is sampled and recorded against the DC voltage, after being amplified, rectified and filtered. For re- 

A is the electrode area, C and D are the concentration and the diffusion coefficient of the electroactive species, AE and co(=2nfj are the amplitude and the angular frequency of the AC applied voltage, t is the time, and j=nF (Edc-Ei/2) / RT. For reversible processes, the AC polarographic wave has a symmetrical bell shape and corresponds to the derivative curve of the DC polarographic wave (Fig. 5.14(b)). The peak current ip, expressed by Eq. (5.24), is proportional to the concentration of electroactive species and the peak potential is almost equal to the half-wave potential in DC polarography  

However, the peak current in AC polarography markedly depends on the reversibility of the electrode process, being very small for an irreversible process. We can apply this dependence to study the kinetics of the electrode reactions. 

The AC current measured in AC polarography contains, in addition to the fara-daic current caused by the electrode reaction, the charging current caused by the 

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Measurements of the interfaeial eapacitance (the differential double layer capacity Cdl) have been used widely, the method has been labelled tensammetry [46Bre, 52Bre, 51Dosl, 52Dosl, 63Bre]. Various experimental setups based on arrangements for AC polarography, lock-in-amplifier, impedance measurement etc. have been employed. In all reports evaluated in the lists of data below the authors have apparently taken precautions in order to measure only the value of Cdl-... 

The term polarography basically refers to a method, where the current flowing across the electrochemical interface is recorded as a function of the applied electrode potential, historically in most cases a mercury electrode is involved. Thus polarography might be called also voltammetry. This sometimes results in confusing terms like e.g. AC voltammetry, which is obviously equivalent to AC polarography (see following entry). (Data obtained with this method are labelled DCP.)... 

The difficulties in conventional polarography as mentioned in Section 3.3.1.1, especially the interference due to the charging current, have led to a series of most interesting developments by means of which these problems can be solved in various ways and to different extents. The newer methods concerned can be divided into controlled-potential techniques and controlled-current techniques. A more striking and practical division is the distinction between advanced DC polarography and AC polarography. These divisions and their further classification are illustrated in Table 3.1. In treating the different classes we have not applied a net separation between their principles, theory and practice, because these aspects are far too interrelated within each class. 

Current-sampled DC and often are used in voltammetry superimposed AC polarography... 

Further, if within the electrical circuit the ohmic resistance R can be neglected, the ic wave leads to the potential by 90°, as is known, which means that shows a positive 7t/2 phase angle shift ( between tt/2 and zero. Our main objective in AC polarography, however, is the faradaic current, so a separating condenser is placed between the amplifier and normal resistor in order to filter out the d.c. current and to evaluate the ac current component. As we want to understand the relationship between idc(i ) and iac(i ) as a function of Edc and Eac applied, we may consider Fig. 3.41(a) and (b). 

Fig. 3.41. Sinusoidal AC polarography. (a) measuring principle, (b) fundamental harmonic ac polarogram (i included).

Fig. 3.42 represents the symmetric bell shape curve of 7, i.e., the genuine fundamental harmonic ac polarogram, which means the curve of only 7F discriminated for 7C, e.g., by means of phase-selective ac polarography. The term "fundamental is related to the character of the polarographic cell as a non-linearized network whose response is not purely sinusoidal but consists of the sum of a series of sinusoidal signals at first harmonic (o>) response, besides that of the second harmonic (2a>), the third harmonic (3a>), etc. 

It has been shown that the charging current if leads to the potential Eac by 90° (cf., eqns. 3.62 and 3.61) and the faraday current if by 45° (cf., eqn. 3.63) these phenomena can be used to discriminate if from if and vice versa by means of phase-sensitive AC polarography. 

Bond and O Halloran42 claim for the combination of phase-selective detection and differential pulse ac polarography a "virtually complete rejection of ic, even at high frequencies and low concentrations . 

Some other less important types of AC polarography may also be considered as sinusoidal ac techniques, as their theoretical treatment can be based on signals from a complex Fourier function in this context we confine ourselves to mentioning sawtooth or triangular wave48 superimposed on the dc ramp. Square-wave polarography is also of that type, but in view of its greater importance we shall treat it separately. 

Tensammetry, although originally a sinusoidal ac polarography, is not limited to that technique, but has been applied also to the Kalousek-type wave form45 and to square waves46 ... 

Fig. 3.47 is comparable to Fig. 3.41 for sinusoidal ac polarography if the tilted shape provides a net compensation of the charging current one obtains a symmetric bell-shaped curve of I in the square-wave polarogram, similar to that depicted in Fig. 3.42. In fact, virtually all of the statements made before on the sinusoidal technique are valid for the square-wave mode except for the rigid shape of its wave this conclusion is according to expectation, especially as Fourier analysis reveals the square wave to be a summation of a series of only...

D. E. Smith, Application of On-Line Digital Computers in AC Polarography and Related Techniques, in J. S. Mattson, H. B. Mark and H. C. MacDonald (Editors), Computers in Chemistry and Instrumentation, Marcel Dekker, New York, 1972. 

The kinetic parameters (Table 9) obtained for the reaction110 are a = 0.04 and k°a = 2.6 x 10 2 cm/s. The rate constant is of the same order as reported by ac polarography.109 The low value of the transfer coefficient indicates that the compound is excessively adsorbed at the electrode surface. 

M. B. Bhargava, AC. polarography of organic compounds, Ph.D. thesis, Bhopal University, 1972. 

The initial electron transfer to form the anion radical species seems to be reversible. For example, Allred et al. investigated the ac polarography of bis(trimethylsilyl)benzene and its derivatives which showed two waves in di-methylformamide solutions [71] the first one is a reversible one-electron wave, and the second one corresponds to a two-electron reduction. Anion radicals generated by electrochemical reduction of arylsilanes have been detected by ESR. The cathodic reduction of phenylsilane derivatives in THF or DME at — 16° C gives ESR signals due to the corresponding anion radicals [5] (See Sect. 2.2.1). 

The double-layer influence on the electrode reaction of Zn(II)/Zn(Hg) on DME in NaNOs solutions was studied in the concentration range from 0.01 to 1 M, using dc and ac polarography [30]. The apparent rate constants of the Zn(II)/Zn(Hg) system increase with dilution of the NaN03 supporting electrolyte. However, after the Frumkin correction, the rate constant was virtually independent of the supporting electrolyte concentration. 

Pournaghi-Aznar MH, Shemirani F, Pourtork S (1995) Electrochemical behavior of some naturally occurring hydroxy derivatives of 9,10-anthraquinone in chloroform at mercury and glassy carbon electrodes application of AC polarography to the analysis of Rhubarb roots. Talanta 42 677-684. 

Bowden, transients, 1401, 1409 Brenner, alloy formation, 1316 Breyer, ac polarography, 1425 Brusic, 1154 Buck, 1121... 

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